This invention relates to a tape cassette, and more particularly to a spring-biased latch for a tape cassette dust door.
For convenience of reference when describing the prior art and the present invention, "inward" means in the direction toward the interior of the cassette from its sides, "outward" means in the direction toward the sides of the cassette from its interior, "upward" means in the direction toward the upper half of the cassette from the lower half, and "downward" means in the direction toward the lower half of the cassette from the upper half.
A tape cassette usually comprises, among other things, a hinged dust door and a latch capable of temporarily locking the dust door closed. The dust door protects the tape in the cassette when the cassette is not being used. The latch is often made of a flat, rectangular lock plate rotatably supported by lateral pivot pins which rest in the lower half of the tape cassette. The lock plate is urged by a torsion spring toward the inward surface of the side wall of the lower half of the cassette. Two square projections on the outward side surface of the lock plate extend through corresponding openings in this side wall of the lower half of the cassette. One of the projections engages an opening in the side of the dust door when the dust door is in the closed position, thereby locking the dust door against opening. When the tape cassette is inserted into a tape player/recorder the latter projection is automatically depressed and the dust door is allowed to open to expose the tape for operation of the tape player/recorder.
The torsion spring used to bias the dust door latch usually has a short and a long leg. The spring is assembled about a shaft which is connected on the top of and parallel to the lock plate. The "legs" of the torsion spring must be correctly assembled in opposing relation to create "torsion" and, therefore, effect biasing of the lock plate normally toward the inward surface of the side wall of the lower half of the cassette, i.e., in the dust door "lock" position. To correctly assemble the torsion spring into the cassette, the spring is usually first pre-loaded about the shaft of the lock plate with the small leg abutting the inward surface of the lock plate, and then the dust door latch must be inserted into the lower half of the cassette to rest on the pivot pins. Alternatively, the lock plate may be inserted into the housing before the torsion spring is positioned on the shaft. In either case, once the lock plate is in place, the long leg must be separately positioned to oppose the force of the short leg. Thus, the long leg must be inserted against the outward side of a vertical wall in the vicinity of the lock plate. Finally, the upper half of the housing is put onto the lower half.
Thus, four assembly steps are normally required for prior art dust door latch assemblies, i.e. assembly of the spring (including positioning the short leg) onto the lock plate, insertion of the lock plate into the housing, positioning the long leg against the housing to oppose the force of the short leg, and, finally, putting the upper and lower halves of the cassette together.
For example, U.S. Pat. No. 4,173,319, issued to Umeda, discloses a torsion spring 45 used to press the lock plate 43 against the inner surface of the side wall of the housing. This spring is mounted to a shaft extending from a pivot pin 44 parallel to the lock plate 43.
The Umeda '319 patent described above is exemplary of the relatively inefficient assembly of dust door latch assemblies, i.e., Umeda signifies the four step assembly of pre-loading the torsion spring on the lock plate, inserting the lock plate into the cassette housing, correctly positioning the long leg of the torsion spring, and joining the two halves of the cassette.